Fix false Valgrind warning.
Fix false Valgrind warning.

On some compiler/platform combination, an assignment of form *p= *p
would map to a memcpy() call, and Valgrind flags this as an overlapped
memcpy() error.

Fix by prefixing *p= *q with if(p!=q) when building for Valgrind
(HAVE_purify).


sql/sql_select.cc:
  Fix false valgrind warning.

flynn1973/mariadb-aix,flynn1973/mariadb-aix,flynn1973/mariadb-aix,davidl-zend/zenddbi,davidl-zend/zenddbi,natsys/mariadb_10.2,ollie314/server,davidl-zend/zenddbi,ollie314/server,natsys/mariadb_10.2,flynn1973/mariadb-aix,natsys/mariadb_10.2,davidl-zend/zenddbi,flynn1973/mariadb-aix,natsys/mariadb_10.2,ollie314/server,ollie314/server,natsys/mariadb_10.2,ollie314/server,flynn1973/mariadb-aix,davidl-zend/zenddbi,davidl-zend/zenddbi,davidl-zend/zenddbi,ollie314/server,natsys/mariadb_10.2,flynn1973/mariadb-aix,flynn1973/mariadb-aix,davidl-zend/zenddbi,flynn1973/mariadb-aix,ollie314/server,ollie314/server,natsys/mariadb_10.2,ollie314/server,flynn1973/mariadb-aix,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,davidl-zend/zenddbi,davidl-zend/zenddbi,davidl-zend/zenddbi,ollie314/server,slanterns/server,ollie314/server,flynn1973/mariadb-aix,natsys/mariadb_10.2